This invention relates to apparatus for limiting the dynamic air pressure or back pressure in an exhaust-type engine supressor or brake, wherein a butterfly-type throttle valve is pivotally mounted in an exhaust pipe, such that the throttel valve can be moved between open and closed positions, and wherein a relief opening is provided for bypassing the throttle valve when the valve is in closed position.
To avoid deleterious effects on an engine equipped with an exhaust-type engine suppressor, it is desirable to limit the maximum back pressure imposed thereby. A known form of pressure lmiting device comprises simply an equalizing opening in the form of a constantly open passage through the throttle valve. However, at low engine rpm, the constantly open passage prevents build-up of adequate engine back pressure and thus may cause a reduction of the pressure under conditions when such reduction is undesirable.